Converting a 4-byte float (IEEE-11073) to a JavaScript number

get-float-value.js

/**
 * Converts a DataView that represents a 4 byte float (IEEE-11073)
 * to an actual float. Useful for example when reading temperature
 * from a Bluetooth thermometer :)
 *
 * The DataView buffer should contain at least 4 bytes:
 *
 *  [b0, b1, b2, b3]
 *   ^   ^   ^   └---------- Exponent
 *   └---└---└------- Will become the mantissa
 *
 * The offset param determines which byte in the DataView to start
 * from.
 *
 * @param value DataView
 * @param offset number
 */
private getFloatValue(value: DataView, offset: number) {
    // if the last byte is a negative value (MSB is 1), the final
    // float should be too
    const negative = value.getInt8(offset + 2) >>> 31;

    // this is how the bytes are arranged in the byte array/DataView
    // buffer
    const [b0, b1, b2, exponent] = [
        // get first three bytes as unsigned since we only care
        // about the last 8 bits of 32-bit js number returned by
        // getUint8().
        // Should be the same as: getInt8(offset) & -1 >>> 24
        value.getUint8(offset),
        value.getUint8(offset + 1),
        value.getUint8(offset + 2),

        // get the last byte, which is the exponent, as a signed int
        // since it's already correct
        value.getInt8(offset + 3)
    ];

    let mantissa = b0 | (b1 << 8) | (b2 << 16);
    if (negative) {
        // need to set the most significant 8 bits to 1's since a js
        // number is 32 bits but our mantissa is only 24.
        mantissa |= 255 << 24;
    }

    return mantissa * Math.pow(10, exponent);
}